1. Field of the Invention
The present invention relates to an optical pickup apparatus configured to perform an operation of reading signals recorded in an optical disc and an operation of recording signals into an optical disc using a laser beam.
2. Description of the Related Art
Optical disc apparatuses are widely used which are capable of performing a signal reading operation and a signal recording operation by irradiating a signal recording layer of an optical disc with a laser beam emitted from an optical pickup apparatus.
The operation of reading signals recorded on a signal recording layer using an optical pickup apparatus is performed by irradiating the signal recording layer with a laser beam emitted from a laser diode and by detecting a change in the laser beam reflected from the signal recording layer using a photodetector.
The optical pickup apparatus includes a polarizing member configured to guide the laser beam emitted from the laser diode toward an objective lens through an operation of allowing the beam to pass therethrough or reflecting it therefrom, and configured to guide return light reflected from the signal recording layer of the optical disc toward the photodetector through the operation of allowing the light to pass therethrough or reflecting it therefrom, and an optical component called a semitransparent mirror or a polarizing beam splitter is commonly used as the polarizing member. A configuration is such that an operation of splitting the laser beam by such a polarizing member is to be performed by utilizing an operation of changing a polarization direction of polarized light using an optical component called a quarter-wave plate, i.e., an operation of converting from linearly polarized light into circularly polarized light and an operation of converting from circularly polarized light into linearly polarized light.
An optical disc including signal recording layers of a plurality of layers instead of one layer, for example, a two-layer optical disc including those of two layers, is recently commercialized, and an optical pickup apparatus, capable of performing an operation of reading signals recorded on the signal recording layers of the optical disc including such a plurality of signal recording layers, is also commercialized.
In optical pickup apparatuses, any one of a three-beam method, a push-pull method, a phase difference method, and applications thereof is employed in accordance with a standard of an optical disc and a type of a medium, as a tracking control method by which the laser beam applied to the optical disc is caused to follow a signal track formed on the signal recording layer.
On the other hand, an astigmatic method and applications thereof are often employed for focus control by which the laser beam irradiating the optical disc is focused on the signal recording layer of the optical disc.
If the differential push-pull method or the three-beam method is employed as the tracking control method and if the differential astigmatic method is employed as the focus control method, the signal recording layer of the optical disc is required to be irradiated with three beams. Thus, the optical pickup apparatus which employs any of these methods includes: a diffraction grating configured to diffract the laser beam emitted from the laser diode into zero-order light and plus and minus first-order diffracted light and form three beams; and a photodetector having light-receiving areas for respectively receiving three reflected light beams which are obtained by reflecting these three beams by the signal recording layer of the optical disc.
FIG. 7 depicts an example of a light-receiving surface of the photodetector including the light-receiving areas where the reflected light beams of the three beams are respectively received.
The photodetector depicted in FIG. 7 includes a main-beam light-receiving portion MD irradiated with a main beam M of zero-order light, a preceding sub-beam light-receiving portion SD1 irradiated with a preceding sub-beam S1 of plus first-order diffracted light, and a following sub-beam light-receiving portion SD2 irradiated with a following sub-beam S2 of minus first-order diffracted light.
In the photodetector, the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2 as described above are arranged in the same straight line, and a configuration is such that a direction in which they are arranged coincides with a tracking control direction, i.e., a direction in which the main beam M, the preceding sub-beam S1, and the following sub-beam S2 respectively applied to the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2 are displaced when a spot condensed and formed by the objective lens is displaced in a radial direction of an optical disc D.
Each of the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2 is configured with a sensor divided into four as depicted. A configuration is such that a signal recorded in the optical disc D is read as a readout signal by performing additions with respect to signals according to the light amounts of the main beam applied to all the sensors A, B, C, and D included in the main-beam light-receiving portion MD.
A focus error signal is generated such that the signals acquired from the sensors in one diagonal relationship in the four-divided sensor included in the main-beam light-receiving portion MD are added, and from such an addition signal, a signal obtained by adding signals acquired from the sensors in the other diagonal relationship are subtracted, and this focus error signal is utilized for performing a focusing control operation. Such a focusing control operation corresponds to a focusing control method called an astigmatic method, which will not be described.
In contrast to the astigmatic method, a configuration is such that a sub-focus error signal is generated such that signals acquired from the sensors in one diagonal relationship in the four-divided sensors included in each of the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2 are added, and from such an addition signal, a signal obtained by adding signals acquired from the sensors in the other diagonal relationship are subtracted, and a focus error signal is generated by performing an arithmetic operation on these sub-focus error signals and a main focus error signal acquired from the four-divided sensor included in the main-beam light-receiving portion MD, thereby performing the focusing control operation. Such a focusing control operation is a control method called a differential astigmatic method.
The focusing control operation by the above differential astigmatic method will then be described. Such a focusing control operation is performed using the main focus error signal and the sub-focus error signal generated from the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2 as described above.
The main focus error signal is acquired such that signals acquired from two sensors A and C of the four-divided sensor included in the main-beam light-receiving portion MD are added, and from such an addition signal, a signal obtained by adding signals acquired from two sensors B and D is subtracted.
A first control signal is acquired such that signals acquired from two sensors I and K of the four-divided sensor included in the preceding sub-beam light-receiving portion SD1 are added, and from such an addition signal, a signal obtained by adding signals acquired from two sensors J and L is subtracted; a second control signals is acquired such that signals acquired from two sensors E and G of the four-divided sensor included in the following sub-beam light-receiving portion SD2 are added, and from such an addition signal, a signal obtained by adding signals acquired from two sensors F and H is subtracted; and the sub-focus error signal is acquired by performing arithmetic processing on the first control signal and the second control signal acquired as such.
A configuration is such that the focus error signal by the differential astigmatic method is acquired by subtracting the sub-focus error signal acquired from the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2 from the main focus error signal acquired from the main-beam light-receiving portion MD.
The operation of generating such a focus error signal will then be described with reference to reference numerals of the sensor units depicted in FIG. 7. If the main focus error signal is denoted by MFE, MFE=(A+C)−(B+D), and if the sub-focus error signal is denoted by SFE,SFE={(E+G)−(F+H)}+{(I+K)−(J+L)}.
The focusing control operation by such a differential astigmatic method is performed based on a differential push-pull signal DPP, and this DPP signal is acquired by DPP=MFE−k1×SFE, where k1 is a constant which is determined based on the light intensity of the main beam and the light intensity of the sub-beams.
Such an optical pickup apparatus has been developed, which is configured to perform the focusing control operation using a combination of the main beam and the sub-beams as described above, and a tracking control operation of the optical pickup apparatus having such a configuration will then be described.
Such a tracking control operation is performed by an operation of irradiating the preceding sub-beam light-receiving portion SD1 with the preceding sub-beam S1 and an operation of irradiating the following sub-beam light-receiving portion SD2 with the following sub-beam S2.
For example, a first control signal is acquired such that signals acquired from the two sensors I and J on the upper side of the four-divided sensor included in the preceding sub-beam light-receiving portion SD1 are added, and from such an addition signal, a signal obtained by adding signals acquired from the two sensors L and K on the lower side is subtracted; a second control signal is acquired such that signals acquired from the two sensors E and F on the upper side of the four-divided sensor included in the following sub-beam light-receiving portion SD2 are added, and from such an addition signal, a signal obtained by adding signals acquired from the two sensors H and G on the lower side is subtracted; and a tracking error signal is generated by performing arithmetic processing on the first control signal and the second control signal acquired as such, however, such an operation is well-known and will not be described.
In addition to the above-described tracking control operation, a method so-called differential push-pull is recently employed in order to improve accuracy, which is a method providing the tracking control operation utilizing a tracking error signal acquired not only from the sub-beams S1 and S2 but also the main-beam light-receiving portion MD irradiated with the main beam M.
A configuration is such that the tracking error signal by this method is acquired by subtracting a sub-tracking error signal acquired from the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2 from a main tracking error signal acquired from the main-beam light-receiving portion MD.
A description is given with reference to the reference numerals of the depicted sensor units. If the main tracking error signal is denoted by MTE, MTE=(A+B)−(C+D), and if the sub-tracking error signal is denoted by STE,STE={(E+F)−(G+H)}+{(I+J)−(L+K)}.
The tracking control operation by such a differential push-pull method is performed based on a differential push-pull signal DPP, and this DPP signal is acquired by DPP=MTE−k2×STE, where k2 is a constant which is determined based on the light intensity of the main beam and the light intensity of the sub-beams.
Such an optical pickup apparatus has been developed, which is configured to perform the tracking control operation using a combination of the sub-beams and the main beam as described above.
The focus error signal and the tracking error signal are generated from the signals acquired from the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2, as described above, to perform the focusing control operation and the tracking control operation based on the focus error signal and the tracking error signal, and the focusing control operation is performed by displacing an objective lens 8 perpendicularly to the signal surface of the optical disc D, and the tracking control operation is performed by displacing the objective lens 8 in the radial direction of the optical disc D.
The optical pickup apparatus is configured as described above, and a description will then be given of a problem caused by return light, i.e., stray light, reflected from an other signal recording layer in a state where an operation of reading signals recorded on one signal recording layer is being performed.
FIG. 7 depicts a relationship between the stray light and the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2, described above, and a portion inside a ring P depicted by a broken line is an irradiation portion of a stray light beam.
As apparent from this figure, the stray light beam spreads in a wide range and is applied onto the main-beam light-receiving portion MD, the preceding sub-beam light-receiving portion SD1, and the following sub-beam light-receiving portion SD2 provided in a photodetector 10.
The zero-order light of the main beam and the plus first-order diffracted light and the minus first-order diffracted light of the sub-beams are generated by a diffraction grating 2, and the light amount ratio thereof, i.e., a light amount ratio of one sub-beam and the main beam is commonly set at about 1:15. Therefore, for example, when the operation of reading signals recorded on a signal recording layer L0 is performed, the light amount of the sub-beams reflected as the stray light from a signal recording layer L1 is sufficiently small as compared to the light amount of the main beam reflected as the stray light, thereby being able to ignore an influence on the focusing control operation, etc.
The light amount of the main beam M applied to the main-beam light-receiving portion MD, i.e., the main beam M reflected from the signal recording layer L0 is sufficiently greater than the light amount of the stray light beam reflected from the signal recording layer L1, thereby not adversely affecting the signal generating operation of the main-beam light-receiving portion MD, i.e., the operation of reading signals recorded on the signal recording layer L0 and the operation of generating the focus error signal.
On the other hand, the light amounts of the preceding sub-beam S1 and the following sub-beam S2, which are applied to the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2 to generate the focus error signal as described above, are set smaller than the light amount of the main beam M. Therefore, a gain of an amplifier provided to amplify the signals acquired from the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2, is commonly set higher than a gain of an amplifier provided to amplify the signal acquired from the main-beam light-receiving portion MD.
As a result, this increases an influence exerted by the stray light, generated from the main beam M reflected from the signal recording layer L1, being applied to the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2. That is to say, since a signal corresponding to the light intensity of the stray light acts on the focus error signal acquired from the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2, the accurate focus error signal cannot be acquired, resulting in unstable focusing control operation, which causes a problem.
The influence of the stray light is also exerted when the tracking error signal is generated from the signals acquired from the preceding sub-beam light-receiving portion SD1 and the following sub-beam light-receiving portion SD2 and the tracking control operation is performed using this tracking error signal, thereby not being able to perform the tracking control operation accurately, which causes a problem.
A technique of forming a shape of a photodetector into such a shape as to eliminate the influence of stray light has been developed as a method of solving the problems caused by stray light (Patent Document 1: Japanese Laid-Open Patent Publication No. 2009-176367). A technique of utilizing a hologram element having polarization selectivity has been developed (Patent Document 2: Japanese Laid-Open Patent Publication No. 2009-76187).
The Patent Document 1 describes a technique of solving the problems caused by stray light by reducing areas of sub-beam light-receiving portions relative to an area of a main-beam light-receiving portion included in a photodetector, however, it is required to work and manufacture with precision, which causes difficulty in manufacturing.
The Patent Document 2 describes a technique of solving the problems caused by stray light utilizing a hologram element having divided areas, however, such a technique uses an expensive hologram element, thereby not only increasing cost but also requiring an adjustment mechanism for adjusting a position of the hologram element.